I. Field of the Invention
The present invention relates to a system and methods generally aimed at surgery. More particularly, the present invention is directed at a system and related methods for performing surgical procedures and assessments involving the use of neurophysiology.
II. Description of Related Art
A variety of surgeries involve establishing a working channel to gain access to a surgical target site. Oftentimes, based on the anatomical location of the surgical target site (as well as the approach thereto), the instruments required to form or create or maintain the working channel may have to pass near or close to nerve structures which, if contacted or disturbed, may be problematic to the patient. Examples of such “nerve sensitive” procedures may include, but are not necessarily limited to, spine surgery and prostrate or urology-related surgery.
Systems and methods exist for monitoring nerves and nerve muscles. One such system determines when a needle is approaching a nerve. The system applies a current to the needle to evoke a muscular response. The muscular response is visually monitored, typically as a shake or “twitch.” When such a muscular response is observed by the user, the needle is considered to be near the nerve coupled to the responsive muscle. These systems require the user to observe the muscular response (to determine that the needle has approached the nerve). This may be difficult depending on the competing tasks of the user. In addition, when general anesthesia is used during a procedure, muscular response may be suppressed, limiting the ability of a user to detect the response.
While generally effective (although crude) in determining nerve proximity, such existing systems are incapable of determining the direction of the nerve to the needle or instrument passing through tissue or passing by the nerves. This can be disadvantageous in that, while the surgeon may appreciate that a nerve is in the general proximity of the instrument, the inability to determine the direction of the nerve relative to the instrument can lead to guess work by the surgeon in advancing the instrument and thereby raise the specter of inadvertent contact with, and possible damage to, the nerve.
Another nerve-related issue in existing surgical applications involves the use of nerve retractors. A typical nerve retractor serves to pull or otherwise maintain the nerve outside the area of surgery, thereby protecting the nerve from inadvertent damage or contact by the “active” instrumentation used to perform the actual surgery. While generally advantageous in protecting the nerve, it has been observed that such retraction can cause nerve function to become impaired or otherwise pathologic over time due to the retraction. In certain surgical applications, such as spinal surgery, it is not possible to determine if such retraction is hurting or damaging the retracted nerve until after the surgery (generally referred to as a change in “nerve health” or “nerve status”). There are also no known techniques or systems for assessing whether a given procedure is having a beneficial effect on a nerve or nerve root known to be pathologic (that is, impaired or otherwise unhealthy).
In spinal surgery, and specifically in spinal fusion procedures, a still further nerve-related issue exists with regard to assessing the placement of pedicle screws. More specifically, it has been found desirable to detect whether the medial wall of a pedicle has been breached (due to the formation of the hole designed to receive a pedicle screw or due to the placement of the pedicle screw into the hole) while attempting to effect posterior fixation for spinal fusion through the use of pedicle screws. Various attempts have been undertaken at assessing the placement of pedicle screws. X-ray and other imaging systems have been employed, but these are typically quite expensive and are oftentimes limited in terms of resolution (such that pedicle breaches may fail to be detected).
Still other attempts involve capitalizing on the insulating characteristics of bone (specifically, that of the medial wall of the pedicle) and the conductivity of the exiting nerve roots themselves. That is, if the medial wall of the pedicle is breached, a stimulation signal (voltage or current) applied to the pedicle screw and/or the pre-formed hole (prior to screw introduction) will cause the various muscle groups coupled to the exiting nerve roots to twitch. If the pedicle wall has not been breached, the insulating nature of the medial wall will prevent the stimulation signal from innervating the given nerve roots such that the muscle groups will not twitch.
To overcome this obviously crude technique (relying on visible muscles twitches), it has been proposed to employ electromyographic (EMG) monitoring to assess whether the muscle groups in the leg are innervating in response to the application of a stimulation signal to the pedicle screw and/or the pre-formed hole. This is advantageous in that it detects such evoked muscle action potentials (EMAPs) in the leg muscles as much lower levels than that via the “visual inspection” technique described above. However, the traditional EMG systems employed to date suffer from various drawbacks. First, traditional EMG systems used for pedicle screw testing are typically quite expensive. More importantly, they produce multiple waveforms that must be interpreted by a neurophysiologist. Even though performed by specialists, interpreting such multiple EMG waveforms in this fashion is nonetheless disadvantageously prone to human error and can be disadvantageously time consuming, adding to the duration of the operation and translating into increased health care costs. Even more costly is the fact that the neurophysiologist is required in addition to the actual surgeon performing the spinal operation.
The present invention is directed at eliminating, or at least reducing the effects of, the above-described problems with the prior art.